docs: def _to_link_header(self, link):
          """
          Convert the link tuple to a link header string. Used internally.
          """
          try:
              bucket, key, tag = link
          except ValueError:
              raise RiakError("Invalid link tuple %s" % link)
          tag = tag if tag is not None else bucket
          url = self.object_path(bucket, key)
          header = '<%s>; riaktag="%s"' % (url, tag)
          return header